U.S. Pat. No. 4,102,664 (Dumbaugh, Jr.) provided the first detailed disclosure of fabricating glass articles having defect-free surfaces which can be equivalent in smoothness to that prepared through grinding and polishing. Thus, the patented process eliminates the need for expensive mechanical grinding and polishing and comprises the following five general steps:
(a) two glass forming batches of different compositions are melted, one batch for a glass which is highly soluble in a given solvent, and a second batch for a glass which is relatively insoluble in the same solvent; PA1 (b) the molten glasses are brought together while in the liquid state, i.e., at viscosities no greater than about 2.5.times.10.sup.5 poises, to form a laminated glass body wherein the insoluble glass is enveloped within the soluble glass; PA1 (c) the glass layers are fused together at a temperature at which they are sufficiently fluid to provide a defect-free interface therebetween; PA1 (d) the laminated article is cooled; and subsequently PA1 (e) the soluble glass layer is dissolved away in an appropriate solvent. PA1 (1) the coefficient of thermal expansion of the soluble glass from room temperature to the set point thereof will not exceed that of the inside glass, preferably it will be somewhat lower; PA1 (2) the viscosity of the soluble glass will be equal to or somewhat lower than that of the inside glass; and PA1 (3) the liquidus temperatures of both glasses will most preferably be below the lamination temperature in order to avoid the onset of devitrification. PA1 (a) A coefficient of thermal expansion compatible with that exhibited by soda lime glass, preferably between 75-95.times.10.sup.-7 /.degree. C. over the range of 25.degree.-300.degree. C. PA1 (b) A solubility in a particular solvent of at least 100 and, preferably, more than 1000 times greater than that of soda lime glass. PA1 (c) Sufficient thermal stability so that it does not devitrify during the forming process. PA1 (d) A composition most preferably free of components which react to any substantial extent with soda lime glass. The patent warns that any reaction taking place between the cladding glass and the soda lime glass must not deleteriously affect the soda lime glass or adversely affect the smoothness of the surface of the soda lime glass when the cladding glass is dissolved away. PA1 (e) A viscosity between 10.sup.4 -10.sup.8 poises, preferably about 10.sup.7 poises, at the temperature at which the laminated glass article is formed. Most preferably, the viscosity of the cladding glass is at least ten times less than that of soda lime glass at the forming temperature. In view of those findings, it was determined that the softening point of the cladding glass comprised a useful reference point. That determination resulted in the ascertainment that the softening point of the cladding glass should range about 575.degree.-650.degree. C. (The softening point of soda lime glass sheet produced via the float process typically ranges about 727.degree. C.). PA1 (a) the linear coefficients of thermal expansion of the two glasses should essentially match; i.e., they will be within about 5.times.10.sup.-7 /.degree. C. and, most desirably, will be within about 2.times.10.sup.-7 /.degree. C. from the setting point of the softer glass to room temperature; PA1 (b) the strain points of the two glasses must be closely similar; i.e., they will be within about 10.degree. C. and, most preferably within 5.degree. C.; PA1 (c) the cladding glass will be at least 100 times more soluble in a particular solvent than the durable glass; PA1 (d) the viscosity of the cladding glass must be at least ten times less than that of the durable glass at the forming temperature; and PA1 (e) the liquidus of the cladding glass should be lower than that of the durable glass. PA1 (1) bringing into contact with each other a body of a glass which is relatively insoluble in an acid solution and a body of a glass consisting essentially, expressed in terms of cation percent on the oxide basis, of 70-85% B.sub.2 O.sub.3, 7-20% BaO, 0-10% PbO, 0-12% R.sub.2 O, wherein R.sub.2 O consists of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, 3-15% PbO+R.sub.2 O, 0-5% Al.sub.2 O.sub.3, 0-6% CaO and 0-7% SiO.sub.2, said glass from its setting point to room temperature having a linear coefficient of thermal expansion within about 5.times.10.sup.-7 /.degree. C. of that of said relatively insoluble glass, a strain point within about 10.degree. C. of the strain point of said relatively insoluble glass, a liquidus temperature below that of said relatively insoluble glass, a viscosity at the forming temperature at least ten times less than that of said relatively insoluble glass, and being at least 100 times more soluble in an acid solution than said relatively insoluble glass, said contact being at a temperature wherein at least said more soluble glass is in fluid form to produce a laminate having an interface between said glass bodies which is substantially free of defects; PA1 (2) cooling said laminate to solidify each glass in fluid state; and thereafter PA1 (3) contacting said laminate with an acid solution to dissolve away said more soluble glass whereby the surface of said relatively insoluble glass from which said more soluble glass has been removed is rendered substantially defect-free and is at least equivalent in smoothness to a conventionally ground and polished glass surface. PA1 (a) both bodies of glass can be in fluid form upon contact; or PA1 (b) the relatively insoluble glass can be in solid form and only the soluble glass is in fluid form upon contact; or PA1 (c) both glass bodies can be in solid form upon contact and at least the contacting (mating) surfaces thereof are exposed to a temperature at which at least the soluble glass is placed in fluid form. As a matter of convenience, the entire laminate will be heated to a temperature sufficient to place at least the more soluble glass in fluid form, but that practice is not mandatory. Localized heating of the mating surfaces, e.g., through laser heating, can be utilized.
As can be appreciated, a most vital feature of the process resides in providing glass compositions exhibiting very different solubilities in a particular solvent. The patent mandates that the outside glass layer be at least 10 times and, more preferably, greater than 100 times more soluble than the inside glass.
In a preferred embodiment the outside and inside glass compositions were selected so that an exchange of ions occurred between the two layers when they were brought into contact with each other and/or when the laminated body was heat treated. That exchange was asserted to enhance the solubility of the outside glass and/or to increase the mechanical strength of the inside glass.
The patent disclosed three general families of glasses suitable for use as the relatively insoluble inside glasses; viz., alkali metal aluminosilicate glasses; alkali metal zinc silicate glasses, and alkali metal, alkaline earth metal (predominantly Sr) silicate glasses. The patent referred to four general families of glasses operable as the soluble glasses; viz., alkali metal silicate glasses, various metaphosphate-type glasses, barium borosilicate glasses, and various borate-based glasses.
With respect to processing parameters for forming the laminated precursor body, the patent mandated the following three requirements:
U.S. Pat. No. 4,880,453 (Coppola et al.) described a modification of the basic process disclosed in U.S. Pat. No. 4,102,664 which was directed expressly to protecting the surface of soda lime glass articles, and providing a surface thereof having the smoothness of a conventionally ground and polished glassware. That is, during the formation of a soda lime glass into an article of a desired configuration, the surface thereof was clad with a glass composition specifically designed to be more soluble in a particular solvent than soda lime glass. The patent set out five vital properties listed below with which the cladding glass had to comply:
Because the glass compositions reported in U.S. Pat. No. 4,102,664 did not demonstrate the necessary combination of melting and forming characteristics to be useful as cladding glasses for soda lime glass, U.S. Pat. No. 4,880,453 disclosed glass compositions expressly devised for that purpose. Those glasses consisted essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ Li.sub.2 O 0.7-7.5 K.sub.2 O 0-5 PbO 18-27 Na.sub.2 O + K.sub.2 O 0-7 Li.sub.2 O + PbO .ltoreq.32 Al.sub.2 O.sub.3 0-5 BaO 35-50 B.sub.2 O.sub.3 30-50 Na.sub.2 O 0-5 SiO.sub.2 5-10 ______________________________________
In the original concept of fabricating glass articles having defect-free surfaces as disclosed in U.S. Pat. No. 4,102,664, the process envisioned cladding the inside durable glass on all sides. In that situation close matching of the thermal expansions of the two glasses is not very critical; it is only necessary that the cladding glass has a thermal expansion essentially the same as, or somewhat lower than, that of the inside glass. Nevertheless, in many applications the durable glass is protected on one side only. Under those circumstances the thermal expansions must be very closely matched, i.e., within about 5.times.10.sup.-7 /.degree. C. from the setting point of the softer glass to room temperature. (As employed herein, a "soft" glass exhibits a low softening point.) Additionally, I have found that the strain points exhibited by the two glasses must be closely similar. Thus, I have observed that, upon cooling the laminated article from the forming temperature, if either glass passes through its transformation range while the other is appreciably more fluid, wrinkling of the harder glass can occur. (The transformation range of glass has been generally defined as the temperature at which a molten mass becomes an amorphous solid, that temperature typically being deemed to reside in the vicinity of the annealing point of a glass.) Finally, as was disclosed in U.S. Pat. No. 4,880,453, the viscosity of the cladding glass ought to be at least one order of magnitude less than that of the durable glass at the forming temperature. In essence, then, each durable glass that is to be fabricated with defect-free surfaces requires a soluble cladding glass that is specifically designed for it.
Therefore, the primary objective of the present invention was to devise a base family of glass compositions wherein, through relatively minor variations in the concentrations of the components, the glasses can perform as cladding glasses for a wide range of durable glasses.
A specific objective of the instant invention was to design a base family of soluble glass compositions suitable for use with durable glasses having coefficients of thermal expansions ranging between about 65-95.times.10.sup.-7 /.degree. C.